1. Field of Invention
This invention relates to enclosure systems associated with sound producing devices and more specifically to vibratorily-driven, sound producing membranes, and more particularly to a device for effectively enlarging the volume of a speaker enclosure of a loudspeaker system in order to increase the compliance that a vibratable cone sees. This device may also permit the varying of the mass of the moving system with respect to frequency.
2. Description of Prior Art
Various sound producing generators, such as audio speakers, operate by driving a membrane in physical vibrations. The vibrating membrane radiates oppositely directed waves in a gaseous medium consisting of alternate regions of increased and decreased pressure. Unfortunately, these front-waves and backwaves can be transmitted through the air to intersect and cause interference, particularly destructive interference.
The more conventional approaches toward the solution of this problem have been to mount the vibratorily-driven membrane at a port provided in the wall of an enclosure. The enclosure is designed either to eliminate the backwaves by absorbing their energy within the enclosure, or by directing the backwaves through passageways and baffles and then transmitting them out of the enclosure in a manner intended to provide only constructive interference with the front waves.
One problem with the first-mentioned solution is that a substantial amount of energy which is used to drive the vibrating membrane is wasted by subsequent absorption in the enclosure. This also cuts the effective efficiency of the sound producer in half by absorbing half of the sound energy produced.
Another problem which increases as a speaker and/or enclosure becomes smaller is that substantial back pressures are exerted against the vibrating membrane by the gas, usually air, within the enclosure.
This back pressure retards the movement of the membrane and results in nonlinear response of the speaker system, especially at lower frequencies where long wavelengths require large membrane excursions. This effect degrades the quality of the sound being reproduced.
A problem with the sound described reflex system is that it is frequency-responsive and consequently constructive interference cannot be uniformly maintained over the broad spectrum of audio frequencies; and, there is a lack of proper mechanical damping of the vibrating membrane, and therefore there is an accompanying degradation of the quality of the sound reproduced.
A problem with both of these designs is that each has a fixed mass in the vibrating system which constrains the designer of the system to make compromises in the design, limiting the width of the response any one design can have.
The less conventional approaches toward a solution to these problems deal with using, in place of air, gases with a [.gamma.] gamma of less than 1.4 which will have an adiabatic compressibility greater than that of air, or liquid.fwdarw.vapor equilibrium systems.
U.S. Pat. No. 3,905,448 entitled Loudspeakers, issued to Hirotake Kawakami, Toshio Sasabe, Toshio Hirosawa, Nobuyuki Arakawa, Kozo Kokubu, Kazumasa Abe and Toshiko Harashino on Sept 16, 1975 teaches a loudspeaker of a general type having a vibratable cone diaphragm.
In Applied Acoustics, H.F. Olsen and F. Massa discuss a back-enclosed cone of a loudspeaker system on pages 197 and 198, the following is an excerpt from that discussion:
"In general, cone speakers are used with both sides of the cone open so that radiation into the air takes place from both sides. For certain uses, as for example a standard source of sound for microphone calibration, reverberation measurement, it is desirable to enclose the speaker mechanism in a box and thus confine the radiation from the cone. The important factor in this system is the stiffness introduced by the box. The net result of this added stiffness is an attenuation of the low-frequency response. A specific example will illustrate the important factors in this system." PA1 "From a consideration of the equation it will be seen that above the resonant frequency the velocity of the system is inversely proportional to the frequency; therefore, since [r.sub.m ] is proportional to the square of the frequency, the power output will be independent of the frequency. Below the resonant frequency the velocity is limited by the compliances [C.sub.m1 ] and [C.sub.m2 ] and the velocity of the cone is practically proportional is rapidly attenuated with decreasing frequency. PA1 Therefore, the low-frequency response limit will be determined by the resonant frequency of the system. PA1 If [C.sub.m2 ] is large compared to [C.sub.m1 ], then the compliance of the box will not materially affect the response and the action will be practically the same as that with both sides open to the air. If the resulting volume when the condition is satisfied is too large and cumbersome, then the system must be altered." PA1 "Such an increase in the effective volume is obtained in adiabatic compression," PA1 "Real gases have non-zero coefficients. This can be anticipated from everyday experience because gas issuing from a small orifice (e.g., a bicycle pump or a compressed-air cylinder) is noticeably cooler than the ambient temperature. PA1 The sign of the coefficient may be positive or negative. A positive sign implies that dT is negative when dp is negative, in which case the gas cools on expansion. A negative sign implies that dT is positive when dp is negative, and so the gas is heated upon expansion. The sign and magnitude of .mu..sub.JT depends on the gas and the conditions. Gases showing heating effects (.mu..sub.JT &lt;0) show a cooling effect (.mu..sub.JT &gt;0) below their inversion temperatures . . . PA1 :using Helium at room temperature would turn a refrigerator into an expensive oven. PA1 For an ideal gas (or a real gas behaving ideally) the Joule-Thomson coefficient is zero."
In the specific example, the velocity of the cone is given by the equation: ##EQU1## where f.sub.m =Bli=b(flux density in the air gap)l(length of wire in the voice coil)i(current in the voice coil), r.sub.m =radiation resistance, m=mass of the cone, voice coil and air load, C.sub.m =C.sub.m1 +C.sub.m2 =C.sub.m1 (compliance of the center and suspension system of the cone)+C.sub.m2 (compliance of the box enclosing the back of the cone).
Another except for Applied Acoustics follows:
This excerpt demonstrates the effect that the compliance of the enclosure has on the position of the resonant frequency of the system and, since it is preferred that a speaker system have a very low-frequency resonance, it is necessarily preferred that a speaker system enclosure have a very high compliance while maintaining a reasonable enclosure volume. The discussion in Applied Acoustics also notes the effect of the mass of the vibrating system on the resonant frequency and states than an increase in mass will result in a decrease in the resonant frequency. Therefore, a large mass is preferred for the low frequencies but a light mass is required for efficient reproduction of higher frequencies; therefore, it is necessarily preferred that the mass of the vibrating system vary with respect to the frequency reproduced.
U.S. Pat. No. 4,101,736 issued to Eugene Czerwinski on July 18, 1978 teaches a speaker enclosure with an enclosed gas; the following is an excerpt:
It is known then that the effective compliance inside a speaker enclosure is proportional to the adiabatic compressibility of the gas therein.
In Mechanics, Heat and Sound by Francis Weston Sears the adiabatic compressibility of an ideal gas is discussed, wherein on page 428 he sets out the following equation: EQU k.sub.AD =1/.gamma.P
where K.sub.AD is the coefficient of adiabatic compression, .gamma. is the ratio C.sub.p /C.sub.v, and p is the pressure of the gas. It is also known that real gases do not obey this law and can deviate greatly from ideal behavior.
In Physical Chemistry by Paul Atkins the deviation from ideal behavior of real gases is discussed on pages 87-88, the following is an excerpt in reference to the Joule-Thomson effect and the Joule-Thomson coefficient .mu..sub.JT :
The formula for the Joule-Thomson effect is: EQU .mu..sub.JT =(.delta.T/.delta.P).sub.H
According to this statement a gas with a Joule-Thomson coefficient of less than zero, undergoing an adiabatic expansion increases in temperature and so, conversely, this gas will cool during an abiabatic compression. This drop in temperature makes up for the increase in pressure incurred in the compression and there is, therefore, no net increase in pressure. Although work is being done and energy is required, the work is approximately linear during this process. The degree to which the Joule-Thomson effect occurs can be graphically demonstrated by referring to a graph of the inversion temperature as a function of pressure and temperature such as those in Heat and Thermodynamics by Zemansky.
The graph of the inversion temperature for the gas Hydrogen in reproduced in FIG. 9, of the drawings. On this PT diagram there is a curve such that, for all values of PT outside the curve .mu..sub.JT is negative, and for all values of PT inside the curve .mu..sub.JT is positive. This PT graph is called an "inversion curve."
Hydrogen and Helium are the only gases readily available with inversion temperatures at or below the ambient temperature of the human environment.
It will be found that a gas with a negative Joule-Thomson coefficient will have a very high adiabatic compressibility, yet will have a relatively low isothermal compressibility; therefore, at mid to low frequencies the compliance of this gas is very high, but at ultra-low-frequencies in which the compression process is isothermal the compliance is very low and the response of the loudspeaker system is limited at infrasonic frequencies. As will be seen in the preferred embodiment of this invention, this is a very advantageous situation when applied to loudspeaker technology.
U.S. Pat. No. 2,797,766 entitled Loudspeaker, issued to H.W. Sullivan on July 2, 1957 teaches an airtight enclosure containing an acoustic diaphragm is provided with a membrane substantially permeable to mechanical vibrations, but substantially impermeable to the gaseous medium on either side of the membrane. The acoustic diaphragm vibrates in a gaseous medium which is heavier than air and in which sound travels at a slower speed than in air; referring back to Mechanics, Heat and Sound on page 498 the following equation is set out: EQU V.sup.2 =(.gamma.)(P/.sub.e)
Where .gamma. is the speed of sound in a gas, .gamma. is as previously described, p is the pressure of the gas, and .rho. is the density of the gas. The characteristic impedance of the diaphragm in the gaseous medium and the acoustical capacitance are lower than those prevailing in air. The difficulty with this design is that the acoustical capacitance of the enclosure still cannot be lowered sufficiently to be considered a preferred speaker enclosure and in a closed enclosure it is the acoustical capacitance which leads to the nonlinear movement of the vibratorily-driven cone or membrane. Also the velocity of sound, being slower than that prevailing in air, leads to highly delayed reflections inside the speaker enclosure, therein degrading the quality of the sound reproduced.
U.S. Pat. No. 4,004,094 entitled Enclosure System for Sound Generators, issued to James Ott on Jan. 18, 1977, teaches a device for use in an enclosure associated with an audio speaker which permits relatively large volume changes within the enclosure as a result of relatively small pressure changes so that relatively small enclosures can be as effective as larger volumes. The device reduces the energy required from the speaker to change the volume of the interior of the enclosure. Pressure perturbations caused by the movement of the vibratorily-driven membrane of the sound producing device cause alternate condensation and vaporization of a composition of matter to minimize back-pressure. The gas-liquid equilibrium is the key to the operation of this device. The patent teaches an improvement in a sound production system which has a less than perfectly sealed enclosure with a flexibly walled container contained therein. The container has a contractable and expandable volume and contains a composition of matter having an equilibrium state between a gas and a liquid phase. Similarly, U.S. Pat. No. 4,101,736 issued to Eugene Czerwinski on July 18, 1978, teaches an improved version of the proceding two patents consisting of a device for enlarging the effective volume of a speaker enclosure including a gas having a gamma .gamma. of less than 1.4 and the product of its density and the square of the speed of sound therein less than the same product for air and a bag which is formed from a soft, pliable material for enclosing the gas within the speaker enclosure and which is adapted to seal the gas therein. The device also includes an acoustically transparent and porous cocoon which is disposed about the bag so that it surrounds completely the bag and an acoustical padding which is disposed adjacent to the sidewalls of the speaker enclosure and which is adapted to enclose the acoustically transparent and porous cocoon. The device may also include a device for generating the gas by heating a fluid in its liquid phase so that the fluid changes to its gas phase. The device is placed in the speaker enclosure in back of the vibratable cone in order to increase the compliance that the vibratable cone sees.
The difficulty with this design is that the preferred speaker enclosure should be unaffected by position, in this design both the surface area of the fluid and its proximity to the heating element therein affect the performance of the device adversely and dictates the necessary orientation, or position, of the enclosure; i.e., upright, sideways, upside down, etc. Another difficulty with this design is the lowered speed of sound in the gas with a gamma of less than 1.4 which leads to delayed reflections from inside the enclosure increasing the inherent distortion and, therefore, degrading the quality of the reproduced sound. Another difficulty with this design (in reference to the preferred speaker enclosure) is the existence of a venting port so that there is a source of high velocity air against the back of the diaphragm of the loudspeaker. Although this design has improved efficiency compared to that of a sealed enclosure, the lack of proper mechanical damping seen by the vibrating diaphragm leads to the generation of a resonance peak over a low-frequency region, which increases the inherent distortion and degrades the quality and accuracy of the sound being reproduced. This lack of proper mechanical damping can lead to very large excursions of the vibrating diaphragm at very low frequencies such as those associated with warped records which can cause eventual breakdown of the vibrating diaphragm, as well as wasting valuable amplifier power on unwanted signals.
There is, therefore, a need for an enclosure system for use with vibratorily-driven sound producing membranes which can effectively dissipate or absorb the backwaves therein without significant pressure variations within the enclosure in order to minimize the input energy required to overcome these backwaves, and to provide a linear effective acoustic resistance in place of the nonlinear acoustic capacitance inherently representative of the interior volume of the speaker enclosure--while raising the compliance of the box to a value practically the same as both sides of the vibratorily-driven membrane being open to the air, and while providing the proper linear mechanical damping of the membrane which will result in a maximally wide, flat response curve over a wide power range and preventing damage to the vibrating diaphragm from unnecessarily large excursions in response to unwanted ultra low-frequency signals. There is also a need for a device containing a gaseous medium in which the speed of sound is as high as is possible so as to bring the time delay of internal cabinet reflections in the mid to high frequency range to a minimum.